Fluid Treatment Tank Having A Laser Welded Distributor Plate

ABSTRACT

A distributor plate for a fluid treatment tank such as a water softener pressure vessel separates the resin bed from a lower end of the resin tank. The outer peripheral surface of the distributor plate is laser-welded to an inner peripheral surface of the tank. The outer peripheral surface of the distributor plate may have a biasing mechanism that biases the outer peripheral surface of the distributor plate against the inner peripheral surface of the tank. The biasing mechanism may include a plurality of peripherally-spaced resilient fingers extending axially and radially away from a circular base of an outer ring of the distributor plate into engagement with the inner peripheral surface of the tank liner, each of the fingers having a base connected to the base of the ring and having a free end. Also disclosed is a method of making a fluid treatment tank.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention generally relates to fluid treatment tanks and, moreparticularly, to a tank that has a fluid-permeable distributor plate andthat can be easily manufactured and assembled. The inventionadditionally relates to such a distributor plate and to a method ofmaking a fluid treatment tank including such a distributor plate.

2. Discussion of the Related Art

Fluid-permeable plates, generally known as “distributor plates,” arewidely used in a variety of fluid treatment tanks. Typical of thesetanks are known as “pressure vessels” in which the fluid being treatedis pressurized, albeit usually at a low pressure such as residentialwater line pressure. One such tank is a pressure vessel known as a“resin tank” of a water treatment system. The typical resin tank is ahollow cylindrical structure the interior of which defines a resin bedconfigured to hold “resin” and water or another liquid therein. Theresin may be provided in the form of a plurality of plastic, e.g.,polystyrene, beads. The resin bed is separated from the bottom of thetank by a slotted, perforated, or otherwise fluid permeable distributorplate that permits bidirectional fluid flow therethrough, but whichprevents resin from falling through the distributor plate. A riser tubemay be centrally positioned within the tank. The typical riser tubeextends from the distributor plate to an upper opening in the tankthrough which treated liquid exits the resin tank. The tank may includea blow-molded plastic tank liner reinforced by an outer layer offiberglass wrap.

Some pressure vessels have distributor plates mounted well-above thebottom of the tank so as to permit a single tank to contain two or morediverse treatment media. Some of upper plates are located in thevicinity of the vertical center of the tank and thus as known as“mid-plate distributor plates” or simply “mid-plates.” Some tankscontain two more such plates located well-above the bottom of the tankin a vertically-spaced relationship. For the sake of conciseness, allsuch plates are referred to herein as “mid-plates”, it being understoodthat the plate need not be located at or near the center of the tank.These mid-plates usually include a cylindrical outer surface that isspin-welded to the interior of the tank's plastic liner. Such plates aredescribed, for example, in U.S. Pat. Nos. 7,901,576 and 8,382,994 toEnpress.

Spin welding involves imparting relative rotation to the components tobe welded to generate sufficient frictional heat to the parts at theirinterface to melt the mating surface of one or both components. Oncerotation stops, the material cools and solidifies and fuses thecomponents together, forming a welded assembly. Spin welding necessarilyinvolves holding the two components in close concentric engagement whilethey are rotated relative to one another. In the case of a watertreatment tank mid-plate, spin-welding typically uses a rotating mandrelor spindle that engages the mid plate while the tank liner is heldstationary. Spin welding a distributor plate to the interior of a tankliner is difficult or impossible unless the tank liner is open near theplate so as to allow insertion of the mandrel axially into the tank. TheEnpress patents discussed herein propose first forming the tank liner,then cutting the tank liner apart in sections to provide an open endnear the mid plate, then spin-welding the mid plate to the inner surfaceof the tank liner, then rejoining the tank liner sections, presumably bythermally bonding them together. The resultant process is time-consumingand equipment intensive. It also risks loss of tank integrity where thecut tank liner sections are rejoined if the welds are imperfect. It alsorisks misalignment of the rejoined sections unless care is taken tomaintain concentricity of those sections during the rejoining process, acondition that can be difficult to meet given the tendency of the linerto become out of round during handling.

The inventor has realized that the need to section tank linerstheoretically could avoided by laser welding the tank liner anddistributor plate together. Laser welding involves directing a laserbeam though a first material that is relatively transparent to the beamto a second material, that is relatively absorbent, so as to heat thesecond material sufficiently to melt and fuse it to first material uponsubsequent cooling. Since the weld energy is imparted through thetransparent outer component, that component need not be open in thevicinity of the inner component to accommodate a rotating mandrel orspindle. However, laser welding is similar to spin welding to the extentthat requires that the components being welded be held in closeengagement with one another during the laser welding process. Theinventor has recognized that, in the case of treatment tank mid plate orother distributor plate, incorporating any measures into a weldingsystem that clamps the plate to the tank liner or otherwise holds theplate and the tank liner in engagement with one another would haveoperate from an open end of the tank liner—just as with a spinwelder—negating a major potential benefit of laser welding over spinwelding.

The need therefore has arisen to provide a mechanism and/or process forholding an outer peripheral surface of a mid-plate or other distributorplate closely adjacent an inner peripheral surface of a water treatmenttank preparatory to and during a laser welding process.

The need additionally has arisen to provide a mechanism and/or a processfor welding a distributor plate in a water treatment tank without havingto leave an end of the tank adjacent the distributor plate open duringthe welding process.

The need additional has arisen to provide a water treatment tank withwelded distributor plate and a seamless tank in the vicinity of thedistributor plate.

SUMMARY

In accordance with a first aspect of the invention, one or more of theabove-identified needs is met by providing a fluid treatment tankcomprising a tank at least one distributor plate that is configured tosupport a bed of a fluid treatment media. The thank has an at leastgenerally cylindrical inner surface, and at least one distributor platereceived in the tank and configured to separate the bed from a lower endof the tank. The distributor plate has an upper surface, a lowersurface, and an outer peripheral surface. The upper surface forms asupport surface for fluid treatment media. A plurality of fluidpermeable openings are formed through the distributor plate from theupper surface to the lower surface for the passage of a liquid beingtreated by the treatment media. The outer peripheral surface of thedistributor plate is laser-welded to the inner peripheral surface of thetank.

In order to facilitate welding of the distributor plate to the tank, theouter peripheral surface of the distributor plate may have a biasingmechanism that biases the outer peripheral surface of the distributorplate against the inner peripheral surface of the tank.

The distributor plate may be formed of two or more components includingan inner fluid permeable disk and an outer ring that supports the diskin the tank. The biasing mechanism may include a plurality ofperipherally-spaced resilient fingers extending axially and radiallyaway from a circular base of the support ring into engagement with theinner peripheral surface of the tank liner, each of the fingers having abase connected to the base of the ring and having a free end. Thefingers may be spaced from one another at the free ends thereof, inwhich case a peripheral seal may extend outwardly from the base of thering and seal against the inner peripheral surface of the tank beneaththe free ends of the fingers. Each of the fingers may be curved along atleast a majority of a length thereof and have an apex between the baseand the free end thereof, the apex engaging the inner peripheral surfaceof the tank.

The tank may be made of a first material that is relatively transparentto laser light of a designated frequency, and at least an outerperipheral portion of the distributor plate may be made of a secondmaterial that is relatively absorbent to laser light of the designatedfrequency.

In accordance with another aspect of the invention, a method of forminga fluid treatment tank that has a distributor plate constructed at leastgenerally as discussed above is provided. The method includes engagingan outer surface of the distributor plate with an inner peripheralsurface of a fluid treatment tank, the distributor plate beingconfigured to separate a bed of treatment media from a lower end of thetank. The distributor plate has an upper surface, a lower surface, andan outer peripheral surface, the upper surface forming a support surfacefor the fluid treatment media, and a plurality of fluid permeableopenings are formed through the distributor plate from the upper surfaceto the lower surface for the passage of a liquid being treated by thetreatment media. The method additionally includes laser-welding theouter surface of the distributor plate to the inner wall of the tank.

The engaging may comprise deflecting a basing mechanism on thedistributor plate against the inner peripheral wall of the tank. In thiseventuality, the engaging may comprise deflecting acircumferentially-spaced resilient fingers against the inner peripheralwall of the tank, the fingers extending radially and axially away from abase of an outer ring of the distributor plate. The distributor platemay have two or more components including an inner disk and an outersupport ring of different materials and that includes the fingers.

Various other features, embodiments and alternatives of the presentinvention will be made apparent from the following detailed descriptiontaken together with the drawings. It should be understood, however, thatthe detailed description and specific examples, while indicatingpreferred embodiments of the invention, are given by way of illustrationand not limitation. Many changes and modifications could be made withinthe scope of the present invention without departing from the spiritthereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the invention are illustrated in theaccompanying drawings, in which like reference numerals represent likeparts throughout, and in which:

FIG. 1 is a sectional side elevation view of a fluid treatment tankincluding a distributor plate according to an embodiment of the presentinvention;

FIG. 2 is a sectional end elevation view of the fluid treatment tank ofFIG. 1, taken generally along the line 2-2 in FIG. 1;

FIG. 3 is a fragmentary isometric view side view of a portion of thepressure vessel, showing the connection of the distributor plate withthe inner peripheral wall of a tank liner of the fluid treatment tank;

FIG. 4 an exploded isometric review of the distributor plate of FIGS.1-3; and;

FIG. 5 is an exploded fragmentary perspective view of the mating ends oftwo ring segments of the distributor plate of FIGS. 1-4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Distributor plates constructed in accordance with the invention could beused in a variety of fluid treatment tanks configured to treat a varietyof fluids using any of a number of media. Hence, while preferredembodiments of the invention now will be described in conjunction with aresin tank of a water treatment system, it is to be understood that theinvention is not so limited.

Referring now to the drawings and, initially, FIG. 1, a resin tank 10includes a tank liner 12 reinforced with wound fiberglass 13 (only aportion of which is shown in FIG. 1). One of variety of water treatmentsystems with which the resin tank 10 is usable is disclosed in U.S. Pat.No. 6,402,944, the contents which are incorporated herein by reference.It should be noted that, in some applications, the tank need not have aninner liner. The term “tank” therefore should be understood to encompasstanks both with and without an in inner tank liner.

The tank liner 12 is made of a thermoplastic material such asblow-molded high-density polyethylene (HDPE). Liner 12 includes agenerally cylindrical hollow body or wall 14 and upper and lowergenerally semi-spherical top and bottom ends 16 and 18. A feature 19 isintegrally molded to the bottom end 18 of the liner 12 to rotationallycouple the liner 12 to a filament winding machine during the fiberglasswinding process The body 14 may have an internal diameter of, forexample, 0.2 m to 1.0 m and a height of 0.5 m to 2 m. The illustratedexample has an internal diameter of about 0.25 m. At least onedistributor plate is provided in the liner 12 to define a treatmentmedia area thereabove. The illustrated embodiment includes two suchplates 20, 22 located near the bottom of the tank 10 and near the centerof the tank 10, respectively and defining respective treatment mediastorage areas 24, 26, thereabove. Each distributor plate 20, 22 hasfluid permeable slots or other openings formed through them that aresized and shaped to permit the relatively free flow of the liquid beingtreated while inhibiting or preventing the ingress of resin beads orother treatment media particles. Each area 24, 26 is configured toreceive a bed or layer of a resin or other water treatment media duringuse of the tank 10. The resin may be provided in the form of a pluralityof plastic, e.g., polystyrene, beads. The same or different treatmentmedia may be received in each area 24, 26. The plate 20 is located nearthe bottom of the tank 10 and can be thought of as a “bottom plate”distributor. The plate 22 is located above the bottom dome 18 and thuscan be thought of as a “mid-plate” distributor plate. A riser tube 28extends vertically through the center of the tank 10 from the beneaththe bottom plate 20, extends through center openings in both plates 20and 22, and extends to or through an upper center upper opening 30 ofthe tank 10. An annular opening 32 is formed between the outerperipheral surface of the riser tube 28 and the perimeter of the opening30. An inlet tube 34, positioned beneath the lower end of the riser tube28, extends downwardly through the bottom distributor plate 20 incommunication with the lower end portion of the tank 10. During a watertreatment operation, untreated water enters the tank 10 from abovethrough the annular opening 32, flows through the media areas 26 and 24and the openings in the associated distributor plates 22 and 20 insuccession, enters the inlet tube 34, and rises out of the tank 10through the riser tube 28. This flow may be reversed, for example,during a treatment media generation cycle.

Pursuant to an implementation of the invention, one or both of thedistributor plates 20 and 22 is mounted in the tank liner 12 by laserwelding. In the illustrated embodiment, the mid-plate 22 is mounted inthe tank liner 12 by laser welding, it being understood that, in otherimplementation\s, the bottom plate 20 and/or still other plates could bemounted in the tank liner by laser welding instead of or in addition tothe mid-plate 20.

With additional reference now to FIGS. 1 and 2, the mid-plate 22 isshown in additional detail. The plate 22 is a two-component assemblyincluding an outer ring 40 mounted on the tank liner wall 14 asdescribed below and an inner disk 42 supported on the ring 40. Thistwo-part distributor plate construction 1) permits the ring 40 to beinjection-molded from a thermoplastic material that is relatively-easilythermally-bonded to the tank liner 12 but that has a relatively lowdimensional predictability, while 2) permitting the disk 42 to beinjection-molded from a thermoplastic material that cannot be easilywelded to the material of the tank liner 12 but that has a relativelyhigh dimensional predictability. “Dimensional predictability” in thisregard refers to a material that can be injection-molded with finefeatures of on the order of a less than a centimeter and as fine as afew tenths of a millimeter with close tolerances and with highrepeatability on a shot-by-shot basis over a relatively large number ofshots. High “dimensional predictability” is generally inversely relatedto shrink rate and is even more closely inversely related to shrink ratevariability. Forming the disk 42 from a material having a relativelyhigh dimensional predictability when compared to that of a material thatcan thermally bonded to the tank liner 12 permits the slots and otherfine features on the distributor plate 20 to be made reliably with closetolerances.

The inner disk 42 of the mid-plate 22 of this embodiment is made fromunfilled or glass-filled Noryl® (Noryl® is a registered trademark ofSabic Innovative Plastics IP B.V.), which is family of amorphousreinforced materials in the form of modified polypropylene ether (PPO)resins that are formed from amorphous blends of PPO resin andpolystyrene. The characteristics of Noryl and other possible inner diskmaterials, as well as the design and construction of such disks, aredescribed in great detail in U.S. Pat. No. 10,106,434, the subjectmatter of which is hereby incorporated by reference in its entirety.

Referring particularly to FIGS. 1-3, the disk 42 includes upper an upperfluid media support surface 46, a lower surface, a central hub portion50 through which the riser tube 28 passes, a cylindrical outer radialedge 52, a plurality of circumferentially-spaced reinforcing ribs 54,and a slotted plate portion 56. An annular plug 57 is received in thehub portion 50. The plug 57 engages the periphery of the hub portion atits outer perimeter and the riser tube 28 at its inner perimeter. Theslotted plate portion 56 is supported on the ribs 54 and extends fromthe hub portion 50 to the outer ring radial end 52. The ribs 54 extendat least generally radially from the hub portion 50 to the vicinity ofthe outer radial edge 52 but stop short of the edge 52 of the disk 42 toform an annular tongue 58 received in an inner groove 60 in the outerring 40.

Referring now to FIGS. 2-5, the above-described openings are provided inthe form of a plurality of slots 62 that are formed in the slotted plateportion 56 of distributor plate 22. The slots 62 are sized, shaped, andorientated to promote uniform fluid flow through the distributor plate22 and the resin without being clogged with the resin beads or othermedia particles. The slots 62 could be relatively straight slots thatextend either radially or tangentially, or could be curvilinear, such asgenerally spiral-shaped. The “slot density” or proportion of the totalarea of all slots 62 to the total area of the slotted plate portion 56also can vary considerably from application to application. In addition,one or more of the slot size, slot shape, slot orientation, and slotdensity can vary across the radius of a given slotted plate portion 56to contribute to a desired flow distribution across the radius of thetank 10. Each slot 62 ideally should be wide enough to permit the fluidbeing treated and other fluids such as brine to pass therethrough butnarrower than the smallest diameter bead or other particle forming thetreatment media, it being understood that the diameters of suchparticles vary greatly and that it is difficult, if not impossible, todesign slots or other openings that prevent any particles from fallingthrough.

Still referring to FIGS. 3 and 4, the tank liner 12 and outer ring 40,or at least those portions of the outer ring 40 that are welded to thetank liner 12, are made of materials that are suitable for thelaser-welding of the ring 40 to the tank liner 12. In order to make thetank liner 12 and ring 40 from as similar of materials as practical, thetank liner 12 may be made from a material of a molecular weight that ison the high end of the range of suitable molecular weights forblow-molding, and the material of the ring 40 is of a molecular weightthat is on the low end of the range of suitable molecular weights forblow-molding that can be injection-molded.

Suitable ring materials are those that are fusible to the tank liner 12and that are relatively absorbent to laser energy of a wavelength foundin laser welders. In the illustrated embodiment in which the tank liner12 is made of a blow-molded, high-density polyethylene (HDPE) that isrelatively transparent to laser energy. One such material is availablefrom Exon Mobile under the PAXON® line and, more specifically PAXONBA53-055. The ring 40 also is preferably made from a high-densitypolyethylene (HDPE). A suitable material is available fromLyondellBasell under the Petrothene® line and, more specifically,Petrothene LT570401. The ring material is impregnated with a material,such as carbon black, that renders it absorbent to laser energy. Thelaser energy absorbent material may be mixed with the HDPE resin priorto the injection molding process in a percentage sufficient to achievethe desired degree of melting during the laser welding process.

Referring briefly to FIGS. 4 and 5, the outer ring 40 is formed of aplurality (three in this embodiment) arcuate ring segments 40A, 40B, and40C connected end to end as described below. Each segment could beformed of two or more interconnected pieces but, in the illustratedembodiment, is formed as a single injection molded component. It thusshould be understood that the discussion of the “ring” that followsrefer to the fully-assembled ring 40 formed by interconnecting the ringsegments 40A, 40B, and 40C.

Referring to FIGS. 2-5, the ring 40 includes a circular base 70 havingan outer peripheral surface, annular horizontal flanges 72 and 74extending radially inwardly from the cylindrical base 70 to form thechannel 60 therebetween, and a spring mechanism 76 for holding thedistributor plate 22 in the tank liner 12 before and during the laserwelding operation, and an annular seal 78. The spring mechanism 76 couldbe formed integrally with or separately from the base 70. It couldcomprise a single bellows or similar structure extending completelyaround the mid-plate 22 or two or more spring elements spacedcircumferentially of one another. In the illustrated embodiment, thespring mechanism 76 comprises fingers 80 collectively extending aroundthe entire circumference of the distributor plate 22. Two (upper andlower) sets of arcuate fingers 80 are provided in this embodiment, withthe sets being symmetrical about a radial bisector of the base 70. Eachset contains 30-40 fingers.

Referring especially to FIGS. 3-5, the fingers 80 of this embodiment aremolded integrally with the remainder of the ring 40. Each finger 80 hasa base 82 merging into the base 70 of the ring 40 and a free end 84located axially and radially apart from the base 82. Each finger 80 isconcave, presenting an apex 86 located between the base 82 and the freeend 84. Each finger 80 engages the inner peripheral surface of the tankliner 12 at least at its apex 86, causing the finger 80 to resilientlydeflect at its base 80 upon engagement of the with the inner wall of thetank liner 12 and impose a retention force on the tank liner 12.Adjacent fingers 80 could be coupled to one another by reduced-thicknessportions of ring material but, in the illustrated embodiment, are spacedby a small gap 88 to enhance the resiliency of the individual fingers 80and to permit one finger 80 to deflect more than an adjacent finger 80,better-accommodating out of roundness, ridges, or other surfaceirregularities in the inner peripheral surface of the tank liner 12. Inthe present embodiment, each finger 80 is 17.5 mm high, measuredaxially, from base to tip, 28 mm wide, and 1.5 mm thick. Each gap 88between adjacent fingers 80 is 0.78 mm thick. In the free state of thefingers, the maximum diameter of the ring 40, as measured at the apices86 of the fingers 80, is greater than the inner diameter of the tankliner body 14 by at least 0.1 cm, more typically on the order of atleast 0.3 cm, and possibly on the order of 0.5 cm or more.

Still referring to FIG. 3, the peripheral seal 78 extends outwardly fromthe base 70 of the ring 40 to seal the ring seals against the innerperipheral surface of the tank liner body 14, preventing fluid andtreatment media from flowing through the gaps 88 between the fingers 80and bypassing the distributor plate 22. The illustrated seal 78comprises a relatively thin annular flange extending radially from theradial bisector of the ring 40 into engagement with the inner peripheralsurface of the tank liner 12. This seal 78 could be formed integrallywith the ring 40 as illustrated or could be mounted in the outerperipheral surface of the ring 40.

As mentioned above, the ring 40 is constructed in segments 40A, 40B, 40Cso as to permit it to be formed around the disk 42 so as to capture thetongue 58 of the disk 42 in the groove 60 of the ring 40. Each segment40A, 40B, 40C extends about 120 degrees around the plate 22. Adjacentsegments could be connected to one another in any desired manner. Asseen in FIGS. 2 and 5, adjacent segments of this embodiment have steppedends that mate with one another at a stepped seam 90. At the matingends, one of the segments has a first, upwardly facing flat surface 92,and the mating segment has a second, downwardly facing flat surface 94.A lug or projection 96 on one of the surfaces is received in a pocket orrecess 98 in the other surface when the adjacent segments are connectedto hold the segments together.

To mount the mid-plate 22 in the tank liner 12, the mid-plate 22 ispre-assembled by connecting the ring segments 40A, 40B, 40C to oneanother with the tongue 58 on the disk 42 captured in the groove 70 inthe ring 40. The plate is then inserted into the cylindrical tank linerbody 14 from one of the ends to the desired position. During thisprocess, the fingers 80 deflect to permit axial movement of themid-plate 22 relative to the tank liner body 14, albeit with someresistance. When the mid-plate 22 is properly positioned in the tankliner body 14, the fingers 80 hold the mid-plate 22 in place, with atleast the apices 86 of the fingers 80 firmly engaging the innerperipheral surface of the tank liner body 14. As mentioned above, fingerdeflection also can accommodate signifying out-out roundness and/orother surface irregularities formed in the inner peripheral surface ofthe tank liner body 14.

The ring 40 then is laser welded to the tank liner body 14 using a laserwelder that transmits laser energy through the tank liner body 14 fromthe outside. A laser suitable for this process is commercially availablefrom Leister under the NOVALS™ Basic AT name. This laser has a 200Wlaser line and operates at a wavelength of 975 nm. The laser energypasses through the tank liner body 14 without appreciably heating it andis absorbed in the ring 40, heating at least the outer portions offingers 80 to above their melting point. When the laser is turned off,the melted material cools, fusing the fingers 80 to the inner peripheralsurface of the tank liner body 14 in the vicinity of the apices 86, andpossibly elsewhere. The mid-plate 22 now is retained in place withsufficient force to support a bed of treatment media and to withstandthe pressures encountered during typical use of the tank 10.

Because laser welding does not require the insertion of a rotatingmandrel or comparable structure into the tank liner body 14, the tankliner body 14 need not be cut in the vicinity of the mounting locationprior to the welding and rejoined after the welding operation. The tankliner body 14 instead can be seamless between the end domes 16 and 18.Fabrication therefore is substantially simplified when compared to thefabrication of a tank having a mid-plate installed by spin-welding. Tankdimensional reliability and tank integrity also are enhanced due to thelack of any seam along the cylindrical portion of the tank liner body14.

Referring again to FIGS. 1 and 2, the bottom-plate 20 also has a disk100 formed Noryl or another material having high dimensionalpredictability, and has slots 102 formed therethrough that may have thesame or similar characteristics of the mid-plate slots 62. Thebottom-plate 20 also could have a ring that is laser-welded to the innerperiphery of the tank liner, likely at a location just above the bottomdome 18, in the same manner as the mid-plate 22. Alternately, the bottomplate 20 could have a ring 18 that has ends that are fusion bonded tofacing ends of the tank liner body 14 and to the bottom dome 18 asdescribed in U.S. Pat. No. 10,106,434, described above and incorporatedby reference in its entirety. However, in the illustrated embodiment, anouter ring is omitted in its entirety. The outer peripheral edge of thedisk 100 instead rests on a support and is prevented from significantupward movement by a weld bead formed by welding the upper end of thebottom dome 18 to the lower end of the tank liner body 14. That weldbead projects over the upper surface of the disk typically bout about0.5 mm to 1.0 mm, preventing significant disk upward surface. Thesupport could be formed from a pan or other structure in the dome 18and/or, as is the case in the illustrated embodiment, by a step or shelf104 provided in or on the upper end portion of the dome 18.

Although the best mode contemplated by the inventors of carrying out thepresent invention is disclosed above, practice of the present inventionis not limited thereto. It will be manifest that various additions,modifications and rearrangements of the aspects and features of thepresent invention may be made in addition to those described abovewithout deviating from the spirit and scope of the underlying inventiveconcept.

For example, distributor plates and tanks as described herein could beused to store fluid treatment media other than resin and could be usedto treat fluids other than water.

In addition, it is conceivable that the entire distributor plate,including the fingers or other biasing mechanism, could be formed of asingle component formed from a material that can be laser welded to thetank liner, in which case the disk and the attendant openings might nothave the same degree of dimensional stability as discussed above.

The scope of some changes to the described embodiments is discussedabove. The scope of other changes to the described embodiments that fallwithin the present invention but that are not specifically discussedabove will become apparent from the appended claims.

We claim:
 1. A treatment tank for a fluid treatment system, comprising:a tank configured to contain a bed of a fluid treatment media and aliquid, the tank having an at least generally cylindrical inner wall; adistributor plate received in the tank and configured to separate thebed from a lower end of the tank, the distributor plate having an uppersurface, a lower surface, and an outer peripheral edge surface, theupper surface forming a support surface for fluid treatment media, aplurality of liquid-permeable slots being formed through the distributorplate from the upper surface to the lower surface, wherein the outerperipheral surface of the distributor plate is laser-welded to the innerwall of the tank.
 2. The treatment tank of claim 1, wherein the outerperipheral surface of the distributor plate has a biasing mechanism thatbiases the outer peripheral surface of the distributor plate against theinner wall of the tank.
 3. The treatment tank of claim 2, wherein thedistributor plate has an outer ring having a circular outer base.
 4. Thetreatment tank of claim 3, wherein the biasing mechanism comprises aplurality of peripherally-spaced resilient fingers extending axially andradially away from the circular base into engagement with the inner wallof the tank, each of the fingers having a base connected to the base ofthe ring and having a free end.
 5. The treatment tank of claim 4,wherein the fingers are molded with the ring, and wherein adjacentfingers are spaced from one another at the free ends thereof.
 6. Thetreatment tank of claim 5, further comprising a peripheral seal thatextends outwardly from the base of the ring and that seals against theinner peripheral surface of the tank beneath the free ends of thefingers.
 7. The treatment tank of claim 4, wherein each of the fingersis curved along at least a majority of a length thereof and has an apexbetween the base and the free end thereof, the apex engaging the innerwall of the tank.
 8. The treatment tank of claim 4, wherein the fingersextend upwardly from the base of the ring, and wherein the biasingmechanism further comprising additional peripherally-spaced resilientfingers extending downwardly and outwardly from the base of the ringinto engagement with the inner wall of the tank liner.
 9. The treatmenttank of claim 2, wherein the distributor plate comprises a disk that issupported on the ring, the disk having the upper and lower surfaces andhaving the slots formed therethrough, the disk being formed of adifferent material than the ring.
 10. The treatment tank of claim 8,wherein the ring comprises at least two arcuate ring segments connectedend to end.
 11. The treatment tank of claim 1, wherein the tank is madeof a first material that is relatively transparent to laser light of adesignated frequency, and wherein and at least an outer peripheralportion of the distributor plate is made of a second material that isrelatively absorbent to laser light of the designated frequency.
 12. Thetreatment tank of claim 11, wherein the outer periphery of thedistributor plate is formed of a HDPE material having carbon imbeddedtherein.
 13. The treatment tank of claim 1, further comprising anotherdistributor plate disposed in the treatment tank beneath the distributorplate.
 14. A treatment tank for a fluid treatment system, comprising: atank liner having an at least generally cylindrical inner wall; firstand second vertically spaced distributor plates disposed in the tank,each of which is configured to supports a respective bed of fluidtreatment media, and each of which has an upper surface and a lowersurface, the upper surface forming a support surface for fluid treatmentmedia, a plurality of slots being formed through each distributor platefrom the upper surface to the lower surface for the passage of a liquidbeing treated by the treatment media, wherein at least one of thedistributor plates has an inner disk which has the upper and lowersurfaces and through which the slots are formed, and an outer ring onwhich the disk is mounted, the outer ring comprising a circular base, afirst set of circumferentially-spaced fingers extending upwardly andoutwardly from the base into engagement with the inner wall of the tankliner, a second set of circumferentially-spaced fingers extendingdownwardly and outwardly from the ring into engagement with the innerwall of the tank liner, and a peripheral seal that extends outwardlyfrom the base and that seals against the inner wall of the tank liner,wherein the tank liner is made of a first material that is relativelytransparent to laser light of a designated frequency, wherein the ringof the at least one distributor plate is made of a second material thatis relatively absorbent to laser light of the designate frequency, andwherein the fingers are laser-welded to the inner wall of the tankliner.
 15. A method comprising: engaging an outer surface of thedistributor plate with an inner peripheral surface of a fluid treatmenttank, the distributor plate being configured to separate a bed oftreatment media from a lower end of the tank, the distributor platehaving an upper surface, a lower surface, and an outer peripheralsurface, and a plurality of fluid permeable openings formed through thedistributor plate from the upper surface to the lower surface for thepassage of a liquid being treated by the treatment media; andlaser-welding the outer peripheral surface of the distributor plate tothe inner wall of the tank.
 16. The method of claim 15, wherein theengaging comprises deflecting a basing mechanism on the distributorplate against the inner peripheral surface of the tank.
 17. The methodof claim 16, wherein the engaging comprises deflectingcircumferentially-spaced resilient fingers against the inner peripheralsurface of the tank, the fingers extending radially and axially awayfrom a base of an outer ring of the distributor plate.
 18. The method ofclaim 17, wherein adjacent fingers are spaced from each other at leastat free ends thereof, and further comprising sealing the base of thering against the inner peripheral surface of the tank at a locationradially inboard from the free ends of the fingers.